Additive effects on the rate of pressure rise for ethylene-air deflagrations in closed vessels

The deflagration of stoichiometric ethylene-air mixtures without or with additives (Ar, N2 or CO2) in spherical and cylindrical closed vessels with central ignition was investigated in order to determine the maximum rates of pressure rise (dp/dt)max and the deflagration indices (severity factors) KG. The influence of initial pressure, additive concentration, shape and size of explosion vessel on (dp/dt)max and KG was studied. A linear correlation between the maximum rate of pressure rise and the total initial pressure was found for all examined systems, at constant initial temperature and fuel/oxygen ratio. The deflagration index KG of centrally ignited explosions, calculated from experimental values of pressure rise rates, was compared with the adiabatic (maximum) deflagration index KG,max calculated from the normal burning velocities and maximum explosion pressures of ethylene-air and ethylene-air-additive mixtures. The differences between experimental and adiabatic deflagration indices highlight the heat losses appearing in the last stage of explosions occurring in cylindrical vessels, strongly influenced by the length to diameter ratio, initial pressure and composition of flammable mixtures.